a) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device which includes a light etching process.
b) Description of the Related Art
Circuit elements such as transistors of a semiconductor integrated circuit device are continuously reducing their sizes to improve the degree of integration. As the sizes of circuit elements become small, contact areas are also reduced. Natural or native oxide films (including chemically oxidized films) are easily formed on a silicon surface if the silicon is exposed in ambient air or reacts with chemicals such as acid. If an electrode or the like is formed on the surface of a contact area having such a native oxide film, the contact resistance increases which may result in a degraded performance or defects of the semiconductor device.
Several methods of solving this problem have been proposed, in which, after a native oxide film is removed from a silicon surface, the silicon surface is terminated with hydrogen atoms to retain the stable state of the silicon surface.
For example, these methods include a dry process using hydrogen plasma (A. Kishimoto et al., Jpn. J. Appl. Phys., 29, 2273, 1990), and a dry process using hydrogen atoms or hydrogen radicals (T. Takahagi et al., J. Appl. Phys., 68, 2187, 1990).
With these methods, however, it is known that a silicon surface may be damaged while a native oxide film is removed and the silicon surface is terminated with hydrogen atoms. Techniques have been long desired for removing a native oxide film and terminating the silicon surface with hydrogen atoms without damaging the silicon substrate and other element regions.
There is also proposed a method, wherein hydrogen- and water vapor-containing gas is excited into a plasma state, and NF.sub.3 is added to a down-flow of the plasma so as to remove a native oxide film by down flow process (J. Kikuchi et al., Jap. J. Appl. Phys., 33, 1994, JP-A-HEI-6-338478, etc.). With this technique, it is possible to remove a native oxide film by using hydrogen radicals and terminate the silicon surface with hydrogen atoms, without damaging the silicon surface.
Experiments made by the present inventors have demonstrated that when a light etching process using a down-flow of hydrogen- and water vapor-containing plasma is carried out at room temperature or above, the etching rate for a deposited silicon oxide film such as a CVD oxide film, a PSG (phosphosilicate glass) film and a BPSG (borophosphosilicate glass) film, becomes higher than the etching rate for a silicon oxide film formed through oxidation of solid silicon such as a thermally oxidated silicon oxide film and a native oxide film.
For example, while a native oxide film on a silicon surface exposed on the bottom of a narrow contact hole formed through a thick BPSG film is removed, the side wall of the contact hole may be excessively etched.